How Now Brown Cloud?

Brown clouds are not the sinister creation of a science fiction writer, but rather the name that atmospheric scientists have given to smog, soot and dirt clouds that obscure the Earth’s surface from space. Smog originates mainly from the burning of fossil fuels for transportation, but can also form naturally in some locations. Soot is found in the atmosphere as a result of wood or coal burning, or natural occurring forest fires. Some people explain these clouds to “pollution clouds,” but in truth these clouds are more complicated than that, sometimes being a result of man’s activities, but other times being wholly natural.

When you spend a lot of time looking at satellite images of the Earth as I do, you get used to certain features of the Earth looking a certain way. Sometimes, these features become obscured by a grey haze. Occasionally this grey haze becomes thicker and darker in color. This is what is referred to generally as a brown cloud. Brown clouds are common over India and Southeast Asia, especially in the late fall through early spring, where rapid industrialization has increased emissions from human activities.


The above figure, taken from NASA’s MODIS website, features Southeastern Asia. To the north of the image is Nepal and China, the mountainous terrain there is crisp and clearly brown in color. As you move south into India and Bengladesh, the surface becomes obscured by a grayish obscuration. The green of India and Bengladesh’s forests is barely visible. This is due to the presence of a so-called brown cloud, a cloud made of soot, smog and dust, most of which are due to human activities.

However these brown clouds are not only native to Asia, but can be found in the United States as well, as the image below shoes.


This satellite photograph, taken from NASA’s MODIS website, shows the Southeastern United States. Here a brown cloud is seen over Louisiana and Texas. The origin of this cloud, seen in the summer of 2004, were extensive forest fires in Southern Canada.

Scientists recently have been examining what exactly causes these brown clouds over Asia to form so regularly each year. Understanding this is important because these brown clouds can affect how much sunlight reaches the surface in these regions, affecting not only climate but crop growth. These clouds are also thought to interact with regular water clouds, changing the way the normal clouds reflect sunlight and how they produce rainfall.

The results of the scientists research are quite shocking, at least to me. The expectation was that a large portion of these brown clouds would come from fossil fuel combustion. The team of scientists report that around 2/3rds of the source material for the brown clouds come from biomass burning. In this region, biomass is used for home heating and cooking, along with burning of residual crops to produce fertilizer. This is an important finding, because it would allow for regional governments to address the issue of unclean home cooking and heating fuels, if they wanted to improve air quality.